Toxins attached to growth factors, antibodies and other cell targeting molecules can be used to kill harmful cells bearing specific receptors or antigens (Pastan et al., Cell 47:641 (1986) and Vitetta et al, Science 238:1098 (1987)). One promising source for an effective therapeutic toxin is Pseudomonas exotoxin A. Pseudomonas exotoxin A (PE) is an extremely active monomeric protein (molecular weight 66 kD), secreted by Pseudomonas aeruginosa, which inhibits protein synthesis in eukaryotic cells through the inactivation of elongation factor 2 (EF-2) by catalyzing its ADP-ribosylation (catalyzing the transfer of the ADP ribosyl moiety of oxidized NAD onto EF-2).
The toxin contains three structural domains that act in concert to cause cytotoxicity. Domain Ia (amino acids 1-252) mediates cell binding. Domain II (amino acids 253-364) is responsible for translocation into the cytosol and domain III (amino acids 400-613) mediates ADP ribosylation of elongation factor 2, which inactivates the protein and causes death. The function of domain Ib (amino acids 365-399) remains undefined, although a large part of it, amino acids 365-380, can be deleted without loss of cytotoxicity. See Siegall et al., Biochem. 30:7154-7159 (1991). PE has been combined with growth factors, antibodies or CD4 to create toxins that can be selectively targeted to cells with different cell membrane proteins as reviewed in Pastan and FitzGerald, Science 254:1173-1177 (1991).
Native PE characteristically produces death due to liver failure. Immunotoxins with PE also attack the liver and, when given in much larger (20 to 250-fold larger) doses, may produce death due to liver toxicity. Improved forms of PE that reduce non-specific toxicity in the host and which improve therapeutic efficacy are highly desirable. Variants of PE omitting the cell binding domain Ia have been found to be effective while reducing the amount of non-specific toxicity. See, U.S. Pat. No. 4,892,827, for example.